A Digital Subscriber Line (DSL) is a data transmission technology using a twisted-pair cable for telephone as a transmission medium. An xDSL, as a combination of the transmission technology, includes High-speed Digital Subscriber Line (HDSL), Single-pair High-speed Digital Subscriber Line (SHDSL), and Asymmetric Digital Subscriber Line (ADSL). Except the SHDSL which uses base-band transmission, other xDSLs that use pass-band transmission can coexist on the same twisted-pair cable with Plain Old Telephone Service (POTS) by using frequency division multiplexing technology.
The twisted-pair cable for telephone adopted in the xDSL technology is used as a transmission channel, and its non-distortion channel capacity must satisfy the Shannon channel capacity formula:
  C  =      B    ·                  log        2            ⁡              (                  1          +                      S            N                          )            
C represents a channel transmission capacity, B represents the signal bandwidth, S represents the signal energy, and N represents the noise energy. It can be seen from the Shannon channel capacity formula that the value of the channel capacity C can be increased by increasing the value of the signal bandwidth B and/or the signal energy S. However, values of both the signal bandwidth and the signal energy have limitations, so that it is difficult to further increase the channel transmission capacity C by increasing the signal bandwidth B or the signal energy S. But from the perspective of the noise energy N, the channel transmission capacity C can be properly increased by lowering the noise energy N.
In practical application, because the pairs of the twisted-pair cables used by the DSL are unshielded, electromagnetic interference among the pairs will result in that the signal on one twisted-pair cable affects the signal on the other twisted-pair cable, that is, a crosstalk problem will be very obvious. Thus, in order to increase the transmission capacity of the xDSL, the crosstalk problem of the xDSL needs to be solved. With respect to the crosstalk problem, in the conventional art, a vector DSL technology is proposed, which neutralizes the crosstalk by using a joint transceiving mode.
In the joint transceiving, a sending filter for joint sending and a receiving filter for joint receiving are mainly implemented according to a crosstalk relationship among a group of lines. When the filters are implemented properly, most crosstalk noises are neutralized, so as to greatly increase a Signal to Noise Ratio (SNR), thereby a channel capacity or a line rate is greatly increased. However, when the filters for joint sending and joint receiving are not properly implemented, the crosstalk noises will be even increased rather than being neutralized. Therefore, in an xDSL system using joint transceiving, the changes of the channel must be continuously monitored during a working process, and the filters for joint sending and joint receiving must be adjusted accordingly, so as to ensure that the filters are always properly implemented. In some cases, the technology is referred to as channel tracking technology. In the current application, channels to be monitored include a direct channel and a crosstalk channel.
The current channel tracking technology is normally applicable to slow changes of the channel. For tracking of a downlink channel, a decided signal error of a receiving end is fed back to a sending end, and then an adaptive technology is adopted at the sending end to update the filter for joint sending, so that the filter for joint sending is adapted to the changes of the channel. For tracking of an uplink channel, a filter for joint receiving is also updated according to a decided signal error feedback, while a slight difference between the tracking of a downlink channel and the tracking of an uplink channel is that an uplink error does not need to be fed back through the channel.
In a process of implementing the conventional art, the inventors find that the prior art at least has the following problems.
A case of an unconventional change of a crosstalk channel is that in a group of lines on which joint transceiving has already been implemented, the impedance of a certain line sometimes will be suddenly changed greatly due to various causes, for example, terminal power off, terminal offline, or broken line, so that the crosstalk channel among the lines instantly undergoes a great change. The change cannot be solved by using the conventional channel tracking technology. Failure to be adapted to the change of the channel within a quite short period of time will result in increasing of line error codes or even offline, which will seriously affect Quality of Service (QoS).